Hydrocarbons – JEE Main ChemistryPractice Questions & PYQs

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The reaction sequence relies on the nucleophilic addition of a Grignard reagent to a nitrile followed by the Clemmensen reduction of the resulting ketone. The addition of methylmagnesium bromide ($CH_3MgBr$) to propanenitrile ($CH_3-CH_2-CN$), followed by acidic hydrolysis ($H_3O^+$), converts the nitrile group into a ketone, yielding butan-2-one ($CH_3-CH_2-CO-CH_3$) as intermediate 'A'.

In the subsequent step, the Clemmensen reduction using zinc-mercury amalgam and hydrochloric acid ($Zn-Hg, HCl$) selectively reduces the carbonyl group ($C=O$) of the ketone into a methylene group ($CH_2$). This process transforms butan-2-one into $n$-butane ($CH_3-CH_2-CH_2-CH_3$), which represents the final product 'B'.

The solvolysis of 2-bromo-3,3-dimethylbutane involves the initial formation of a secondary carbocation, $CH_3-CH^+-C(CH_3)_3$, which spontaneously rearranges via a 1,2-methyl shift to a more stable tertiary carbocation, $CH_3-CH(CH_3)-C^+(CH_3)_2$. Elimination reactions occurring from these two distinct carbocations lead to the formation of three alkenes: 3,3-dimethyl-1-butene, 2,3-dimethyl-1-butene, and 2,3-dimethyl-2-butene. Additionally, the nucleophilic substitution of these carbocations by a solvent molecule, such as water, produces two isomeric alcohols, 3,3-dimethyl-2-butanol and 2,3-dimethyl-2-butanol, resulting in a total of five distinct chemical products.

The ring expansion process is governed by the thermodynamic stability of the resulting six-membered cyclic systems, which significantly reduces ring strain compared to the initial five-membered rings. During the rearrangement, migration of the alkyl group to the electrophilic center occurs in both cyclic components, driving the transformation of both rings into their more stable six-membered configurations.

The addition of $HBr$ to an unsymmetrical alkene proceeds via an electrophilic mechanism following Markovnikov's rule in the absence of peroxides, where the proton adds to the carbon with more hydrogen atoms to form the more stable carbocation intermediate, subsequently leading to the formation of the alkyl bromide at the more substituted position. In the presence of organic peroxides, the reaction follows a free-radical chain mechanism, known as the peroxide effect or Kharasch effect, where the bromine radical attacks the terminal carbon to form the more stable alkyl radical, resulting in the Anti-Markovnikov product with the bromine at the less substituted position. Consequently, the distinct reaction pathways dictated by the reaction conditions determine the regioselectivity, yielding the respective major products $A$ and $B$.

The chlorination of 2,2,5,5-tetramethylhexane ($C_{10}H_{22}$) involves replacing a hydrogen atom with chlorine, a process governed by the molecule's internal symmetry and the distinct environments of its hydrogen atoms. The twelve hydrogen atoms located on the four equivalent methyl groups are identical, so substitution at any of these sites yields only one achiral monochloro derivative. The methylene hydrogens at the central positions are also equivalent to each other, but replacing one of these hydrogen atoms creates a chiral center, resulting in a pair of distinct enantiomers. Because the achiral methyl-substituted product and the two chiral methylene-substituted enantiomers are all unique, the maximum number of isomeric monochloro derivatives is 3.

The reaction proceeds via the nucleophilic addition of the Grignard reagent $CH_3MgBr$ to the electrophilic carbonyl carbon of the ketone 3-methyl-2-butanone. The methyl carbanion attacks the carbonyl carbon to form an alkoxide intermediate, which subsequently undergoes acid hydrolysis with $H_3O^+$ to yield the tertiary alcohol 2,3-dimethylbutan-2-ol as the major product. This addition follows the standard mechanism for organometallic reactions with carbonyl compounds, where the carbon-magnesium bond breaks to provide a nucleophilic carbon that selectively binds to the carbonyl center.

The synthesis follows a sequence of catalytic aromatization, selective oxidation, and disproportionation to produce the desired organic compounds. n-Heptane undergoes cyclization and dehydrogenation in the presence of $\mathrm{Cr}_2\mathrm{O}_3$ at $770\,\text{K}$ and $20\,\text{atm}$ to form toluene, which serves as the intermediate $Q$. Toluene is subsequently oxidized via the Etard reaction using $\mathrm{CrO}_2\mathrm{Cl}_2$ followed by $\mathrm{H}_3\mathrm{O}^{+}$ to yield benzaldehyde as the intermediate $R$. Finally, because benzaldehyde lacks $\alpha$-hydrogens, it undergoes the Cannizzaro reaction upon treatment with $\mathrm{NaOH}$ followed by acidic workup with $\mathrm{H}_3\mathrm{O}^{+}$ to produce benzoic acid.

The major product in this elimination reaction is determined by Zaitsev's rule, which states that the most substituted alkene is the thermodynamically favored product. When a strong base like $\text{NaOEt}$ reacts with an alkyl halide, an $E2$ mechanism occurs, where the removal of a $\beta$-hydrogen from the most substituted carbon leads to the formation of the most stable, highly substituted double bond. This pathway minimizes steric hindrance while maximizing hyperconjugation and bond stability, thereby resulting in the most substituted alkene as the major product.

The reaction sequence begins with the hydroboration-oxidation of propene ($\mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}_2$) using $\mathrm{B}_2\mathrm{H}_6$ followed by $\mathrm{H}_2\mathrm{O}_2$ and $\mathrm{NaOH}$, which results in the anti-Markovnikov hydration of the alkene to yield propan-1-ol ($\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_2\mathrm{OH}$). Subsequent treatment with Pyridinium chlorochromate ($\mathrm{PCC}$) selectively oxidizes this primary alcohol to the corresponding aldehyde, propanal ($\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CHO}$), without further oxidation to a carboxylic acid. Finally, the nucleophilic addition of the Grignard reagent $\mathrm{CH}_3\mathrm{MgBr}$ to the carbonyl carbon of propanal produces a magnesium alkoxide intermediate, which upon acidic workup yields the secondary alcohol, butan-2-ol ($\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_3$).

Hydrogenation reactions reveal the number of double bonds in an organic compound because each pi bond requires one mole of hydrogen to become saturated. Dividing the 17 mg mass of the hydrocarbon by its molar mass of 136 g/mol gives $1.25 \times 10^{-4}$ moles. With a volume of 8.40 mL of $H_2$ gas at standard conditions, the ideal gas law equation $n = \frac{PV}{RT}$ determines that $3.75 \times 10^{-4}$ moles of hydrogen are consumed. Comparing the moles of $H_2$ to the moles of the hydrocarbon ($C_{10}H_{16}$) yields a ratio of 3, confirming that the molecule contains 3 double bonds.

The Wolff-Kishner reduction utilizes hydrazine ($NH_2NH_2$) and a strong base, such as potassium hydroxide ($KOH$), to convert an aldehyde or ketone into the corresponding alkane by reducing the carbonyl group ($>C=O$) to a methylene group ($>CH_2$). Benzaldehyde is successfully reduced by this method to form Toluene, meaning any assertion claiming it cannot undergo this reaction or produces a different product is false. The provided reason is true as it accurately describes the fundamental utility of the Wolff-Kishner reduction in converting carbonyl functionalities to methylene units.

The reaction of an aldehyde with hydroxylamine ($NH_2OH$) proceeds via a nucleophilic addition-elimination mechanism to form an oxime. The lone pair on the nitrogen atom of the hydroxylamine acts as a nucleophile, attacking the electrophilic carbonyl carbon of the aldehyde, which is followed by the elimination of a water molecule to stabilize the structure with a carbon-nitrogen double bond of the form ($R-CH=N-OH$).

Ozonolysis of an alkene involves the oxidative cleavage of the $C=C$ double bond to produce two carbonyl-containing fragments, which can be identified by removing the oxygen atoms from the products and reconnecting the carbon atoms with a double bond. The provided products, ethanol (specifically acting as ethanal, $CH_3CHO$) and propanone ($CH_3COCH_3$), yield the fragments $CH_3CH=$ and $=C(CH_3)_2$ upon removing the carbonyl oxygen atoms. Connecting these fragments forms the structure of the hydrocarbon $(X)$ as 2-methylbut-2-ene, which has the chemical formula $C_5H_{10}$. The molar mass of the hydrocarbon is determined by summing the atomic masses of all constituent atoms: $(5 \times 12) + (10 \times 1) = 60 + 10 = 70 \text{ g mol}^{-1}$.

The conversion of a trans-alkene to its cis-isomer requires proceeding through an alkyne intermediate to control stereochemistry during reduction. Bromination with $\mathrm{Br}_2$ followed by double dehydrohalogenation using $\mathrm{alc\ KOH}$ and $\mathrm{NaNH}_2$ yields the internal alkyne $\mathrm{Ph-C}\equiv\mathrm{C-CH}_3$, while the use of $\mathrm{aq\ KOH}$ is unsuitable as it promotes substitution rather than elimination. Partial hydrogenation of this alkyne with $\mathrm{H}_2$ and Lindlar catalyst facilitates syn-addition to stereoselectively produce the cis-alkene, whereas reduction with $\mathrm{Na\ (Liq\ NH}_3)$ would yield the trans-isomer.

The electrophilic aromatic substitution of ethylbenzene with $Br_2$ and $Fe$ yields 1-bromo-4-ethylbenzene as the major product because the alkyl group is an ortho/para director and the para position is sterically favored. Subsequent radical halogenation using $Cl_2$ and heat occurs selectively at the resonance-stabilized benzylic position, converting the ethyl group into a 1-chloroethyl substituent. The final treatment with alcoholic $KOH$ performs an $E2$ dehydrohalogenation, which eliminates $HCl$ from the alkyl chain to form the conjugated alkene, 1-bromo-4-vinylbenzene ($p$-bromostyrene).

The reaction sequence relies on the generation of a nucleophilic acetylide anion followed by $S_N2$ alkylation and a stereoselective partial reduction. Treatment of the terminal alkyne with $n\text{-BuLi}$ abstracts the acidic acetylenic proton to form the acetylide intermediate, which subsequently attacks $n\text{-pentyl chloride}$ to produce the internal alkyne, $\text{CH}_3(\text{CH}_2)_3\text{C}\equiv\text{C}(\text{CH}_2)_4\text{CH}_3$. The application of Lindlar's catalyst with $\text{H}_2$ gas facilitates a partial hydrogenation that proceeds via syn-addition across the triple bond, yielding the cis-alkene isomer as the major product.

Alkenes are inherently less stable than alkanes because they contain a $\pi$-bond, which is weaker than the robust $\sigma$-bond characterising alkanes. Even so, the carbon-carbon double bond remains stronger than a single bond in terms of bond energy, since breaking both the $\sigma$ and $\pi$ bonds necessitates more energy than breaking a solitary $\sigma$ bond.

The synthesis hinges on a two-step sequence that first installs a hydroxyl group via anti-Markovnikov addition and then selectively oxidizes it. Hydroboration-oxidation, employing $\mathrm{BH}_3$ followed by $\mathrm{H}_2\mathrm{O}_2 / \mathrm{OH}^-$, adds water across an alkene to generate a primary alcohol at the less sterically hindered position. PCC then serves as the essential mild oxidizing agent, stopping the reaction at the aldehyde stage to yield product B from the intermediate alcohol derived from A.

The interaction between a chlorine radical and a methane molecule is a fundamental hydrogen abstraction step in free radical chlorination. During this collision, the highly reactive chlorine radical captures a hydrogen atom from the methane, which results in the formation of hydrogen chloride and a reactive methyl radical. This mechanism is defined by the following equation:

$\mathrm{Cl}^\bullet + \mathrm{CH}_4 \rightarrow \dot{\mathrm{CH}}_3 + \mathrm{HCl}$